Simple cleanups of DictFieldData

I tried to make DictFieldData strict about only being used with one
block.  That effort failed ultimately, but these are reasonable changes
I made along the way.

lms/djangoapps/open_ended_grading/utils.py

@@ -12,8 +12,6 @@ from django.conf import settings
 
 from mitxmako.shortcuts import render_to_string
 
-from xblock.field_data import DictFieldData
-
 import logging
 
 log = logging.getLogger(__name__)